Re: SFINAE, specialization, trait selection and problems

"Greg Herlihy" <>
21 Jan 2007 15:50:57 -0500
Bjorn Fahller wrote:

I think I may have gone over board on this one, but I have a template, for
which I can provide good default behaviour for all built in integer types,
and for random accessible containers of built in integer types, and
especially so for strings.

So, below is a nasty piece of code showing the thoughts for this. I'm sure
it's possible to get the functionality in more elegant ways. The code works
with G++ 4.1.1 and Intel C++ 9.1, although I had to resort to an #ifdef due
to different ideas between the two. Comeau online is very unhappy about the
whole thing and complains violently (it strongly disagrees with the
T::iterator thing.)

Which is right, if any? What's a better way of achieving the same thing?

Sorry for the longish code, but I have problems making a shorter example.

I think it should possible to simply your implementation. First, take
advantage of std::tr1::type_traits (soon-to-be, std::type_traits) for
the integral type test. Second, specialize the is_string test for
std::string and std::wstring (any other string type can add their own
specialization). Third, use a simplified SFINAE test for the
"randomly-accessbile container of integral types":

Applying these suggestions produces the following implementation:

     #include <string>
     #include <iostream>
     #include <limits>
     #include <vector>
     #include <list>
     #include <map>
     #include <set>
     #include <deque>
     #include <iterator>

     #include <tr1/type_traits>

     #include <boost/static_assert.hpp>

     using std::tr1::is_integral;

     template <class T>
     struct is_string
         static const bool value = false;

     template <>
     struct is_string< std::string >
         static const bool value = true;

     template <>
     struct is_string< std::wstring >
         static const bool value = true;

     template <class T1, class T2 = std::random_access_iterator_tag>
     struct is_random_access_container_of_ints
         static const bool value = false;

     template <class T>
     struct is_random_access_container_of_ints<
                 typename T::iterator::iterator_category >
         const bool value = is_integral<typename T::value_type>::value;

     template <class T>
     void Check()
             is_integral< T >::value or
             is_string< T >::value or
             is_random_access_container_of_ints< T >::value);

     #define CHECK(t) Check<t>()

     int main()
        // These should all work:

        CHECK( int );
        CHECK( std::vector<int> );
        CHECK( std::wstring );
        CHECK( std::deque<int> );
        CHECK( std::string );

        // These must cause compilation errors if uncommented:

        // CHECK( std::vector< std::string > );
        // CHECK( std::list<int> );
        // CHECK( double );
        // CHECK( name );


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